The present invention relates to non-aqueous lithium cells, such as storage batteries.
Non-aqueous lithium cell storage batteries typically include an anode of metallic lithium, a lithium electrolyte prepared from a lithium salt dissolved in one or more organic solvents and a cathode of an electrochemically active material, typically a chalcogenide of a transition metal. During discharge, lithium ions from the anode pass through the liquid electrolyte to the electrochemically active material of the cathode where the ions are taken up with the simultaneous release of electrical energy. During charging, however, the flow of ions is reversed so that lithium ions pass from the electrochemically active material through the electrolyte and are plated back onto the lithium anode.
During each discharge/charge cycle small amounts of lithium and electrolyte are consumed by chemical reactions at newly created surfaces. As lithium inherently tends to form high surface area peaks or dendrites as it is plated back onto the anode, this reactive condition is aggravated. Furthermore, the dendritic peaks continue to grow until they eventually contact the cathode which causes the cell to fail. Additional amounts of lithium do not cohesively plate onto the anode during the charge cycle and result in the formation of spongy deposits near the anode surface. As these deposits are not in electrically conductive contact with the anode, they eventually detract from the capacity of the cell.
One approach to minimizing the consumption of lithium is to prevent the growth of lithium dendrites and spongy deposits so that only a low surface area layer is deposited. One method of accomplishing this is to provide a sheet-like porous separator on the lithium surface and apply substantial pressure on the separator, and hence on the anode. Typically, this pressure is applied as an inter-electrode pressure, also referred to as "stack pressure". This approach minimizes the dendritic and spongy growths of the lithium, and helps to insure that a low surface area plating is deposited. However, only cells with cylindrical symmetry can be made to withstand this large pressure with a thin metal casing. Rectangular and coin-shaped cells would require very thick casings in order to withstand this pressure without excessive flexing, thereby resulting in a larger battery and increased cost.
Only very expensive separators are available which are porous yet prevent dendritic penetration by lithium, and which are able to withstand the very large cell pressures which are developed. Even with these separators, however, there is a risk that the separator will be punctured by dendritic growth, so that only long recharge times may be used. Also, low discharge rates increase the chances of dendritic separator puncture during charging, thereby limiting the number of charge/discharge cycles which may be obtained.
Even when porous separators and stack pressure are used, a small percentage of lithium is still consumed during each discharge/charge cycle. Thus, in order to attain a practical cell life, it is necessary to include a substantial excess of lithium in the cells, thereby significantly increasing their cost and size.
Moreover, lithium metal is extremely reactive and has a low melting point. With lithium cells of large size there is a danger that the heat generated during abnormal cell operation may lead to melting of the lithium anode. Such melting would not only render the cell inoperative, but could also lead to direct contact between the molten lithium and the cathode material, resulting in a vigorous reaction that could rupture the cell casing.
In addition, the use of lithium metal as the anode material usually requires that a toxic salt, LiAsF.sub.6, be used in the electrolyte in order to obtain optimum cell performance. The LiAsF.sub.6 apparently contributes to formation of coatings on the lithium which enhances the performances of the cell. The use of this toxic substance, however, presents danger both during manufacture and in those situations where the cell casing may rupture.
Thus, there exists a need for a rechargeable cell which will provide the advantages provided by cells having lithium metal anodes, but which will not have the drawbacks associated with these types of cells. One approach has been to replace the lithium metal anode with a carbon anode such as coke or graphite intercalated with lithium metal to form Li.sub.x C. In operation of the cell, lithium passes from the carbon through the electrolyte to the cathode where it is taken up just as in a cell with a metallic lithium anode. During recharge, the lithium is transferred back to the anode where it reintercalates into the carbon. Because no metallic lithium is present in the cell melting of the anode cannot occur even under abuse conditions. Also, because lithium is reincorporated in the anode by intercalation rather than by plating, dendritic and spongy lithium growth cannot occur.
This technique, however, has encountered numerous problems. As Li.sub.x C is a reactive material which is difficult to handle in air, it is preferably produced in-situ in a cell. In doing so, however, lithium and cell electrolyte are consumed in an irreversible process. This irreversible process results in an initial capacity loss for the cell which reduces the cell's overall performance. Another problem with this approach is that the cell exhibits a progressive loss of capacity over numerous charge/discharge cycles. This progressive loss is commonly referred to as "capacity fade".
Accordingly, there are still needs for further improvements in cells having carbon electrodes.